French President Nicolas Sarkozy delivers a speech at the Elysée Palace in Paris on Monday. (Jacques Brinon/Associated Press)

French President Nicolas Sarkozy has vowed revenge for the killing of a 78-year-old French aid worker in the Sahara over the weekend.

Michel Germaneau's killers "will not go unpunished," Sarkozy said Monday, adding the killing illustrated the need to keep up the fight against terrorism.

Sarkozy also defended his country's decision to take part in a failed operation with Mauritania to free Germaneau, in which six al-Qaeda members died.

"Convinced he was condemned to a certain death, we had the duty to make this effort to pull him free from his captors," Sarkozy said in a public address after an emergency government meeting in Paris on Monday.

That effort failed and Germaneau was killed "in cold blood," Sarkozy said, without specifying when or where.

Germaneau was abducted April 22 in Niger, and officials later said he was taken to Mali.

European newspapers had reported the military raid took place last Thursday.

Killers slam Sarkozy

Amid increasing concerns about terrorism and trafficking in northwest Africa, four countries — Algeria, Mauritania, Mali and Niger — in April opened a joint military headquarters deep in the desert. The goal has been to establish a collective response to threats from traffickers and the branch known as al-Qaeda in the Islamic Maghreb.

Al-Qaeda in the Islamic Maghreb had given France until Monday to help secure the release of its jailed members in the region, warning that Germaneau would be executed if Paris failed to comply.

"As a quick response to the despicable French act, we confirm that we have killed hostage Germaneau in revenge for our six brothers who were killed in the treacherous operation," the group's leader, Abdelmalek Droukdel, said in a message broadcast on Al-Jazeera television.

"Sarkozy has [not only] failed to free his compatriot in this failed operation, but he opened the doors of hell for himself and his people," he added.

Before retiring, Germaneau worked in the Algerian oil sector. He later ran an aid group that worked in African countries called Enmilal. The association couldn't be immediately reached for comment on his slaying.